In the last several years there has been an increasing interest in zirconia ceramics. This interest emanates largely from the discovery by Garvie, et al that partially stabilized zirconia (PSZ) ceramics could be fabricated with high strength and high fracture toughness. Garvie named the material "ceramic steel". Current applications include extrusion and drawing dies, cyclone heads, and tool bits. A large number of wear applications are envisaged. The largest potential application is in automotive engines where partially stabilized zirconia is being tested as a material for piston crowns, valve guides, and valve seatings. In the advanced diesel engines, currently under development, partially stabilized zirconia is the leading candidate material for the cylinder liner and piston cap.
The term stabilized zirconia as used herein refers to both fully and partially stabilized zirconia materials. Fully and partially stabilized zirconia can be fabricated with a number of dopants or stabilizers. Examples of these are yttria, calcium oxide, or magnesium. The quality of the powder and distribution of dopant in the powder play an important role in determining the microstructure of the ceramic after sintering. Since the effectiveness of the martensitic transformation responsible for stabilized zirconia's strength and fracture toughness depends upon its microstructural features, control of the physical and chemical properties are essential. The following factors affect the martensitic transformation in PSZ:
(1) Grain size and grain size distribution PA0 (2) The transformation temperature PA0 (3) Type and concentration of stabilizing agent PA0 (4) Density of the sintered ceramic PA0 (5) Purity and the presence of grain boundary phases PA0 (6) Microstructural homogeneity PA0 (7) Twin spacing PA0 (8) Phase type and concentration.
Many of these factors can be related to the characteristics of the starting powder. For example, if the particle size of the starting powder is too large the strain energy in the grain will be sufficient to convert tetragonal grains spontaneously to the stable monoclinic form. A similar effect will occur if the ceramic does not have a high density due to lack of constraint by the surrounding matrix. This may also occur near porous areas in high density materials. Such effects are due to the presence of agglomerates in the starting powder.
The purity of the powder may affect the sintering characteristics of the powder as well as retention of the tetragonal phase through the formation of grain boundary phases which aid in sintering, but lower the high temperature properties of the ceramic as well as reduce the concentration of the stabilizer in the grains comprising the bulk of the ceramic. Use of a high quality powder is thus an essential prerequisite for producing optimum PSZ ceramics.
The objectives of microstructure control in any ZrO.sub.2 -containing ceramic are (a) to obtain as high a volume fraction of the tetragonal particles as possible, and (b) to optimize the particle size and size distribution. Large particles transform spontaneously and do not contribute to toughening while very small particles will require very high stresses for transformation. It is desirable to have a narrow particle-size distribution about the optimum size. This can be achieved by the present invention.
There are five main methods of producing partially stabilized zirconia powder: (1) powder mixing, (2) coprecipitation and decomposition, (3) vapor phase decomposition, (4) sol-gel processing, and (5) hydrothermal processing. The preferred method of preparing stabilized zirconia powder is the one that gives the best combination of cost and performance in terms of the cost of producing the powder and the powder's technical features. Mixing commercially available powders is an inexpensive method to prepare the powder, but may result in ceramics with poorer properties because solid state mixing does not always result in homogeneous distribution of the dopant throughout the powder.
A popular method for preparing stabilized zirconia powder is coprecipitation and decomposition in which salts of both the zirconia and the stabilizer are first precipitated from solution. This mixture of salts is then calcined to form the oxide. Fine reactive powders can be prepared by this method, but the calcination step requires a high temperature step to produce the oxide and may create agglomerates in the powder. Furthermore, the grinding operation required to break down the agglomerates can contaminate the powder. The vapor phase decomposition process is a thermal (or plasma assisted) chemical vapor decomposition reaction in which chlorides or metallorganic compounds of zirconia and the stabilizer are used as the starting materials. Powders made by this technique are generally extremely fine and difficult to handle because of their low bulk density. The process is also relatively expensive. In the sol-gel process alkoxides are polymerized and subsequently heat treated to form the oxide. Ultrafine powder is generally produced, but with appropriate engineering a wide variety of particle sizes can be prepared; however, unless excess water is avoided, the powder may contain chemically bound water which prevents the particles from being fully dense. The raw materials for this process, alkoxides of zirconia and yttria, are also relatively expensive. The method has the advantage that partially stabilized powder (not a mixture of zirconia and the oxide of the stabilizing agent) is produced directly.
The hydrothermal method of preparing zirconia powder in the present invention offers the best possibility of producing a high quality partially or fully stabilized zirconia powder at attractive production costs.
The objectives of the invention are to produce a high quality stabilized or partially stabilized zirconia powder from zirconyl nitrate, from zirconyl oxychloride, from zircon sands and other appropriate feedstocks, with controlled particle sizes that can be used to fabricate ceramics with superior properties, to produce a lower cost powder, and to produce a reactive free flowing powder exhibiting a high degree of crystallinity and crystalline perfection, a high degree of homogeneity and containing little or no bound water.